The present invention relates to controllable-pitch aircraft propellers, particularly to such propellers which are adapted to be operated with engines having a beta pitch-control mechanism, such as the Pratt & Whitney PT 6A engine series. More particularly, the invention relates to featherable and reversible turbo propellers whose low pitch stop is established by a beta control valve.
The propeller control system to which the present invention is applied is basically the system as shown in U.S. Pat. No. 3,249,159, issued May 3, 1966, to Biermann and U.S. Pat. No. 3,446,289, issued May 27, 1969, to Morris, Jr., in which the blade position is transmitted to a valve (a beta valve) through a mechanical feedback arrangement carried on the propeller. Conventionally, the beta valve is not modulated when the throttle control is advanced from the idle position to the full throttle position; in fact, in conventional propeller pitch control systems, the beta control valve is only modulated when the throttle control is moved from the idle position to the reverse position or vice versa. The propeller control system can also be of the type having an internal feedback to a beta rod such as shown in Biermann, U.S. Pat. No. 2,986,222, issued May 30, 1961 or Biermann, U.S. Pat. No. 3,380,535, issued Apr. 30, 1968.
Recent improvements in propeller design have resulted in the development of quieter, more efficient propellers having multiple blades, e.g. 4 or more. Although these propellers reduce noise and improved performance, they suffer from the drawback that the increased surface area of the propeller due to the increased number of blades results in a propeller that produces more drag than propellers having fewer blades.
In multiple-engine airplanes, the FAA requires that the airplane shall not fly below an established minimum control speed (V.sub.MC). The V.sub.MC is defined as the minimum speed required to maintain directional heading when the critical engine is producing no power and the other engine is at maximum power and RPM. When higher drag multiple-bladed propellers are retrofitted onto existing airplanes, the V.sub.MC of the retrofitted airplane increases to values considerably higher than that which the airplane was originally certified. These increased V.sub.MC values normally result in the recertification of an airplane at a V.sub.MC l that requires an unacceptably high take-off speed and field length. Therefore, in order to avoid the need to recertify the retrofitted aircraft and obtain the benefits of the multiple-bladed propellers, it is desirable to try and satisfy the original certification using the multiple-bladed propellers.
Previously, in order to satisfy the V.sub.MC requirements of the FAA, airplanes that were retrofitted with the quieter and more effective multibladed propellers required an automatic feather system. The automatic feather system senses reduced torque or horsepower output from the critical engine, while the throttle is requiring full power, and automatically features the propeller into the propeller's lowest drag position (blade streamlined with line of flight). By minimizing the drag of the propeller of the critical engine, the yawing moment around the airplane's vertical axis is reduced. In order to maintain a directional heading when the critical engine is without power, the rudder of the airplane must be used to provide a countering moment to the yawing moment. The ability of the rudder to avoid the yawing moment is a function of the speed of the airplane; as the yawing moment increases (i.e., increased drag on propeller of critical engine), the speed of the airplane must increase to enable the rudder to counteract the yawing moment. Thus, by minimizing the drag of the critical engine, the yawing moment is reduced, and accordingly, the airplane speed necessary to counteract the yawing moment and maintain a directional heading (i.e., the V.sub.MC) is reduced to its lowest airspeed value; unfortunately, the automatic feathering systems are complex and costly to install and introduce an additional source of potential malfunction.
Therefore, in airplanes capable of being retrofitted with the state of the art multibladed propellers, as well as new aircraft using multibladed propellers, it would be desirable to provide a system that could replace the costly automatic feather devices and enable the airplane to maintain a V.sub.MC close to that for which is was originally certified or as low a V.sub.MC as can be obtained to help optimize takeoff and flight performance on airplanes of new design. The system should be simple to install and should provide substantially the same propeller blade angles as conventional systems when the airplane is at ground idle or in reverse.